CHAPTER 2 LITERATURE REVIEW
2.4 Methods for Landslide Hazard Assessment
2.4.1. Qualitative Methods
Qualitative methods are commonly known as direct methods. Qualitative methods include use of index or parameter maps and field geomorphology analysis approaches. Qualitative approaches are entirely based on the site specific-experience or judgment of experts. These approaches mainly require aerial photogrammetric images and field survey data as supporting data for image interpretation use. High resolution satellite images may complement required spatial data. More detail about qualitative methods can be found in Aleotti and Chowdhury [116], Suzen [57], Guzzetti [36], and Van Westen [108].
2.4.1.1 Field Geomorphology Analysis
This method is also called as heuristic method, landslide distribution mapping, and experience-based method. This method is considered as the most straight forward qualitative method to create LHZ map as stated by Wieczorek [119]. Hazard assessment is carried out directly in the field by the experts of earth sciences based on their experience. Suzen [57] stated that this method requires identification of mass movements and assessment of the geomorphologic conditions for constructing LHZ map. The method also employs interpretation of aerial photo or high resolution satellite images. Another required data may come from database of historical landslide occurrences.
The advantage of this method is that the hazard map can be constructed directly after obtaining the required spatial data. The assessment of the stability at a particular area can be carried out quickly by involving a large number of causative factors. In Addition, this method can be applied to all scale as suggested by Aleotti and Chowdhury [116] and Suzen [57]. However, this method has disadvantages as reported by Aleotti and Chowdhury [116] as follow:
1. The spatial data selection to construct landslide hazard map and the rules that control slope stability are relied on the subjectivity of the investigators or experts. Hence, comparison between hazard maps produced by different
investigators is difficult to do due to the difference subjectivities of the investigators.
2. Subjectivity of the method. It means that there is no explicit rule so that critical analysis of the result is difficult to do. Another problem in assessment of the result may arise when new data become available.
3. This method is laborious because it requires lengthy field surveys.
Examples of mapping landslide hazard zones using geomorphology field analysis are mostly found in literatures of the 70s and 80s. Some recent investigations using this method can be found in Mantovani, et al. [120] who conducted geomorphologic survey at Olvera area, Spain; Hiramatsu, et al. [121] who carried out geomorphologic analysis using LiDAR (Light Detection And Ranging), Hearn [122] who worked on geomorphologic mapping of Ok Tedi copper mine, Papua New Guinea, and other investigations carried out by Canuti, et al. [123], Lee and Talib [55], and Mantovani, et al. [124]. The final product is a map showing spatial distribution of mass movement either shown as coverage (areas) or point symbols.
Suzen [57] stated that in most LHA methods, landslide distribution or inventory map is used as a base landslide map. By using this map, the relation between landslide and causative factors can be extracted. This relationship would provide useful information for applying statistical methods.
2.4.1.2 Overlay or Combination of Index Maps or Parameter Maps
To construct a landslide hazard map using this method, a set of landslide causative factors is selected by an expert based on his/her experiences. Each factor has classes.
The expert assigns an appropriate weighted value to each factor; a value that is proportional to its relative contribution to cause slope stability. According to Soeters and van Westen [125], this method requires the following procedures to complete:
1. Division of each causative factor into a number of relevant classes.
2. Assignment of a weighted value to each class.
3. Assignment of a weighted value to each causative factor.
4. Overlay of the weighted thematic maps/causative factors.
5. Production of the landslide hazard map representing level of hazard.
Reviewed by Aleotti and Chowdhury [116], this method has advantages that the role in determining hazard level is explicit even though it still contains the subjectivity from the expert opinion in assigning weighted values. This method also enables automation using GIS and standardization of data management from data collection to analysis. In addition, it can be applied to various scales. However, this method is time consuming when applied for a large area. Another drawback of this method is that the subjectivity of the expert still exists in term of attributing weighted value for each causative factors and classes. Extrapolating a model built in a certain area to other areas faces difficulties.
Some examples of application of this method can be found in Anbalagan [2], Anbalagan and Singh [126], Abu-Zeid, et al. [127], Turrini and Visintainer [128], and Ramli, et al. [93]. The problem related to defining numerical weighted values has been overcame by Anbalagan [2] by introducing the first rating system called Landslide Hazard Evaluation Factor (LHEF) to evaluate the relative significance of each causative factor. Anbalagan [2] also developed a role to evaluate relative importance between classes /sub categories of a causative factor. To apply LHEF rating scheme, Liao [47] suggested the following procedures: 1) evaluating the relative importance between causative factors based on their influence to slope instability in the area of study; 2) evaluating the relative importance between classes of a particular causative factor according to their significance in contributing slope failure; 3) constructing a thematic for each causative factor showing weight values for pixels; 4) summing up all thematic map to produce the final LHZ map.
LHEF rating scheme was first applied to study landslide hazard in the mountainous terrain of Himalaya by Anbalagan [2]. The main causative factors included in this scheme were lithology, structure, slope gradient, relief, land use land cover, and ground water condition. The selection of these factors was based on field observation at the study area. Table 2.2 shows the maximum ratings/weight values of each factor. The rating was determined using empirical approach based on the author experience obtained from the field study about the relation between landslide
causative factors and their influence on landslide occurrences. The maximum LHEF value for lithology, structural discontinuities, slope gradient, land use land cover was set to 2.0 while the remaining factors were set to 1.0.
Table 2.2 LHEF rating for causative factors
Causative Factor Maximum LHEF
Lithology 2.0
Structural discontinuities 2.0
Slope gradient 2.0
Relative relief 1.0
Land use and land cover 2.0
Ground water conditions 1.0
Source: Anbalagan [2]
For awarding weight values for causative factor classes, Anbalagan [2] applied an experience based subjective assignment. For example, land use land cover was subdivided into five classes: agriculture, thick forest, moderate forest, sparse forest, and barren land. Barren land was considered as the most unstable land cover and given a weight value of 2.0. Thickly vegetated forest areas were considered as the most stable areas and assigned a weight value of 0.8. The remaining classes were given weight values in this way, based on the experience of the investigator. The rating values of all causative factors along with their correspondence classes are presented in Table 2.3
The final landslide hazard map indicated the total estimated hazard (TEHD) which was the total summation of the weight values of all factors, i.e. lithology, structure, slope gradient, relief, land use land cover, and ground water condition.
TEHD expressed the net probability of slope instability. Based on TEHD values, the final hazard zones map was presented in five categories namely very low hazard (TEHD<3.5), low hazard (3.5-5.0), moderate hazard (5.1-6.0), high hazard (6.1-7.5), and very high hazard (>7.5). The maximum of TEHD value may change as the number of factors involved in constructing final hazard map increases or decreases.
Table 2.3 LHEF Rating System
Factor Subcategories/Classes Rating
Lithology: rock type
Quartzite and limestone 0.2
Granite and gabbro 0.3
Gneiss 0.4
Well-cemented terrigeous rocks 1
Poorly-cemented terrigeous rocks 1.3
Slate and phyllite 1.2
Schist 1.3
Shale with interbedded clayey rocks 1.8
Highly weathered shale 2
Lithology: soil
Old well compacted fluvial fill material 0.8
Clayey soil 1
Sandy soil 1.4
Debris 1.2
Old well compacted young loose material
2
Structure: depth of soil cover
< 5 m 0.65
6-10 m 0.85
11-15 m 1.3
16-20 m 2
>20 m 1.2
Slope gradient
>45 2
36-45 1.7
26-35 1.2
16-25 0.8
<15 0.5
Relative relief
<100m 0.3
101-300m 0.6
>300 1
Land use land cover
Agriculture 0.65
Thickly vegetated forest area 0.8
Moderately vegetated forest area 1.2 Sparsely vegetated forest area 1.5
Barren land 2
Water conditions
Flowing 1
Dropping 0.8
Wet 0.5
Damp 0.2
Dry 0
Source: Anbalagan [2]